Mosses Use Explosive Cannons to Spread Spores

Image 1: A bed of sporulating Sphagnum moss. This moss reproduces by shooting spores out of a round capsule at the tip of their stalks. In this image, the tips of the stalks that are spherical have an intact cap and have yet to explode. The capsules that have already exploded are cylindrical or lack a cap.

Image 2: A still from a 9-frame series at 0.1 millisecond intervals showing a Sphagnum moss capsule explosion. Researchers used ultra high-speed cameras to reveal that each launch is accompanied by a miniature mushroom cloud, indicating the formation of a vortex ring. These rolling haloes of air give the spores the extra boost they need to get high enough off the ground to catch the wind currents that carry them long distances.

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 To reproduce, Sphagnum, a type of peat moss, must disperse spores into the air that are carried by the wind over long distances. But Sphagnum live as a flat mat low to the ground. So how do they send their spores high enough into the air to what's called the "turbulent boundary layer" (a zone 10 centimeters off the ground), where swirls of air and sideways currents can carry the spores over long distances?

Researchers Dwight Whitaker of Pomona College and Joan Edwards of Williams College discovered that in order to accomplish this task, Sphagnum shoot spores out using their stalks as cannons. Anywhere from 20,000 to 30,000 spores are "loaded" into a round capsule at the tip of the stalk. On sunny days, the capsule dehydrates and collapses inward, transforming from a sphere into a cylinder and squashing the air inside it. After a while the pressure builds up until it is so great that the capsule blows its top, shooting out both spores and air like a cannon. The spores are ejected at around 30 miles per hour with around 32,000 times the force of gravity, allowing them to reach the turbulent boundary layer above the moss. The entire process takes less than a hundredth of a millisecond.

Whitaker and Edwards were awarded a National Science Foundation major research instrumentation grant (grant DBI 07-22532) for Williams College to establish a high-speed imaging facility. The facility enables researchers to view things in the natural world that occur in the blink of an eye, like the strike of a mantis shrimp or the sprint of a greyhound dog.

Whitaker and Edwards used the facility's ultra high-speed cameras--which shoot up to 100,000 frames per second--to film the firing of the spore cannons. They discovered that each launch is accompanied by a tiny mushroom cloud. The clouds are actually rolling haloes of air called vortex rings, that give the spores the extra boost they need to reach the turbulent boundary layer.

To learn more about this research, see the Discover Magazine story “Mosses use explosive cannons and mushroom clouds to spread their spores.”

(Date of Image: 2010)

Credit: Joan Edwards